The invention applies more particularly to the coding of interlaced frames, that is to say those composed of two fields sampled at distinct instants.
The coding of video frames is necessary for the transmission of digital video data in limited bandwidth networks as well as for applications where storage constitutes a limit.
The compression of digital video data without significant degradation of quality is possible when the video sequences display a high degree of spatial redundancy, of spectral redundancy, of temporal redundancy, of psycho-visual redundancy.
In the course of the last few years, interest in respect of multimedia has led to the emergence of new compression standards and has seen the development of standards such as MPEG-2 and more recently H.264 defined under the reference: ITU-T Rec. H.264|ISO/IEC 14496-10 AVC.
These standards are aimed at a vast gamut of applications with varied demands with regard to bit rate, to frame quality, to complexity, to resilience to error and to timescale as well as to the improvement of compression ratios.
The H.264 compression standard has introduced, mirroring what was done in MPEG-2, the notion of frame coding structure interlacing the lines of the frame or field separating two fields of the frame.
The field or the frame consists of horizontal rows, each containing sixteen lines of pixels. In each row there are macroblocks, each being of dimensions 16*16. A macroblock is organized as four luminance blocks and as 2, 4 or 8 chrominance blocks depending on the type of sampling.
In a compression scheme formatting the data in frame mode in a macroblock, the two fields remain interlaced in a common block. In the field type coding mode, the two fields are separated in the macroblock: one block corresponds to the even field and another corresponds to the odd field.
The H.264 standard also includes the known types of frame defined in the MPEG-2 standard, namely:                I (intra) frames whose coding does not depend on any other frame,        P (predictive) frames whose coding depends on previously received frames,        B (bi-predictive) frames which depend on frames received previously and/or subsequently. The group of frames between two I frames is called a GOP (standing for “Group of Pictures”), its size is variable and its internal structure in terms of frame type is also free.        
The reference model of the H.264 standard has made the choice of so-called a-posteriori coding, that is to say after post-coding of the frame in the frame or field structures. The coding which provided the best bit rate/distortion compromise is adopted.
Such a mode of coding is hardly conceivable for real-time applications since it is too expensive in terms of time or calculation means: it necessitates the calculation of frame structure field predictions, frame structure frame predictions, field structure field predictions, i.e. three times as many calculations as the coding of a progressive source (frame structure frame predictions) which is already critical for a standard television format in the current state of the art.
Moreover, the reference model of the standard implements a fixed GOP structure, this not guaranteeing that the choice of coding, although made in a-posteriori mode, will make it possible to obtain the best coding in terms of bit rate/distortion compromise.